Effects of Vitamin C Supplementation on Glycemic Control and Cardiovascular Risk Factors in People With Type 2 Diabetes: A GRADE-Assessed Systematic Review and Meta-analysis of Randomized Controlled Trials
PURPOSE To investigate the efficacy of oral vitamin C supplementation in improving glycemic control, cardiovascular risk factors, and oxidative stress in people with type 2 diabetes.
DATA SOURCES Databases (PubMed, Embase, Scopus, Cochrane Library) and clinical trial registries were searched for randomized controlled trials up to 09/08/2020.
STUDY SELECTION Trials in adults with type 2 diabetes were included. Trials were excluded if supplements were not exclusive to vitamin C, and if <2 weeks in duration.
DATA EXTRACTION Primary outcomes were HbA1c, glucose, cholesterol, triglycerides, and blood pressure (BP). Data were extracted for changes in outcomes between vitamin C and control groups. Evidence certainty was assessed using GRADE methods.
DATA SYNTHESIS Twenty-eight studies (n=1574) were included in the review. Outcomes changed to a statistically and clinically significant extent with Vitamin C were systolic BP (mean difference [MD] −6.27, 95% CI [−9.60, −2.96] mmHg; p=0.0002) with moderate evidence certainty; and HbA1c (MD −0.54 [−0.90, −0.17] %; p=0.004) and diastolic BP (MD −3.77 [−6.13, −1.42] mmHg; p=0.002) with very low evidence certainty.
LIMITATIONS Studies were predominantly short-term (<6 months) with a small number of participants (n<100).
CONCLUSION While evidence from short-term studies suggests vitamin C supplementation may improve glycemic control and blood pressure in people with type 2 diabetes, vitamin C supplementation cannot currently be recommended as a therapy until larger, long-term and high quality trials confirm these findings.
